Duchenne muscular dystrophy (DMD) is a fatal, x-linked disease that causes progressive muscle weakness and susceptibility to damage. DMD is caused by a lack of dystrophin, a large muscle protein that performs both structural and signaling functions. A promising treatment currently in clinical trials is microdystrophin gene therapy, which delivers a truncated version of dystrophin to muscle via a viral vector. Preclinical studies have established efficacy of microdystrophin to improve muscle quality and function. With clinical success of this treatment, patients affected by DMD could become more physically active. However, the effect of exercise on both dystrophic and gene therapy-treated muscles is unclear. Recently, we demonstrated that microdystrophin gene therapy with and without 21 weeks of voluntary wheel running (VWR) improved treadmill time to fatigue and in vivo plantarflexor torque output in young mdx mice, a mouse model of DMD. Although treated mice could run well, diaphragm force and power output were blunted by VWR. A subsequent study tested longevity of two different microdystrophin gene therapy constructs in combination with VWR. Versions of each construct are being tested in clinical trials. Construct 1 contained the nNOS-binding site found in full-length dystrophin, which localizes nNOS to the sarcolemma and reduces functional ischemia of exercising limb muscles, while construct 2 lacked the nNOS-binding site and was the same microdystrophin used in the previous study. Gene- therapy treated mice that were sedentary or performed 52 weeks of VWR demonstrated similar outcomes including increased plantarflexor torque and exceptional treadmill endurance capacity. However, ex vivo diaphragm and soleus force, as well as metabolic enzyme and mitochondrial respiration assays were differentially improved, revealing unique physiological adaptations to each microdystrophin construct. Together, the data demonstrated that response to exercise after gene therapy treatment was variable and dependent on age, microdystrophin construct, and muscle type. / Doctor of Philosophy / Duchenne muscular dystrophy (DMD) is a rare, fatal muscle disease that causes progressive muscle weakness and cardiorespiratory failure. Available treatments, such as corticosteroids, slow progression of the disease but do not address the underlying genetic cause. DMD is caused by a genetic mutation that results in the loss of the muscle protein dystrophin. Microdystrophin gene therapy aims to address the genetic cause of the disease by using a non-pathogenic virus to deliver microdystrophin, a small, functional version of dystrophin, to muscle. This gene therapy is in clinical trials, and, if it is successful, treated patients will likely want to engage in more physical activity than previously possible due to muscle weakness. However, the effects of physical activity on muscles treated with gene therapy are unclear. Therefore, we conducted two studies to test the effects of voluntary wheel running on microdystrophin gene therapy in the mdx mouse, a model of DMD. The first study demonstrated that voluntary wheel running was beneficial to whole-body muscle function in mice treated with microdystrophin gene therapy. However, adaptations to the gene therapy and voluntary wheel running were variable in individual muscles. In the second study, we tested two microdystrophin constructs, which each contain different structural components of full-length dystrophin. In addition, mice ran for 52 weeks, more than twice as long as the first study. The results of the second study found that adaptations in individual muscles depended on microdystrophin construct and activity level. Additionally, we confirmed that voluntary wheel running was beneficial to whole-body function of microdystrophin–treated muscles. Together, these studies demonstrated that adaptations of gene therapy-treated muscles were dependent on microdystrophin structure, activity level, and age.

The purpose of this thesis project was to elucidate the immune transcriptomic changes in the diaphragm of mdx mice treated with microdystrophin gene therapy with and without running wheel activity. Mdx mice are a model of Duchenne Muscular Dystrophy (DMD). Similar to DMD, mdx pathophysiology is associated with chronic inflammation due to sarcolemma fragility and cellular membrane leakage. Immune modulation has not yet been described when endurance exercise and AAV-microdystrophin gene therapy have been combined in mdx mice. An increase of physical activity in DMD individuals is a potential outcome of current clinical studies investigating microdystrophin treatment; therefore, understanding the impacts of physical activity on the immune system, particularly for the diaphragm, may be important to minimize risk. Recently, the Grange lab published the endurance and contractile property outcomes of combined microdystrophin gene therapy and running wheel activity in mdx mice.1 Diaphragm RNA-seq transcriptomic data were also collected from this study for gene expression analysis. Using this dataset, I tested the hypothesis that relative to mdxGT (mdx mice treated with gene therapy), transcripts related to the immune response such as immune cell recruitment, activation, and downstream signals that promote fibrosis deposition were unchanged or downregulated in mdxRGT (mdx mice treated with gene therapy and access to running wheel). DEGs (differentially expressed genes) were analyzed with Microsoft Excel, R, and bioinformatic tools such as KEGG and DAVID to explain immune system adaptations in response to combined microdystrophin treatment and running in mdx mice. Two major inflammatory signaling pathways, the IL-6/JAK/STAT and NF-kB signaling pathways translationally relevant to DMD patients were rescued by gene therapy towards WT expression levels. Although running maintained the majority of the rescued transcriptome profile (691 of 724 genes), some immune response-related gene expressions (33 of 724 genes) were modulated including genes related to chemotaxis and cellular migration. These changes suggested potential signaling for angiogenesis and a fast to slow fiber type shift; however, unbiased analysis with bioinformatic tools did not confirm either of these possibilities. The data from this study revealed inflammatory and fibrotic signaling pathways commonly observed in DMD patients and mdx mice were rescued by the AAV microdystrophin gene therapy and were maintained by voluntary wheel running / Master of Science / Duchenne Muscular Dystrophy (DMD) is an X chromosome-linked muscular dystrophy, a genetic disease that affects around 1 in 14,000 boys globally. DMD is lethal and currently there is no cure. Mutations in the DMD gene results in the absence of the protein dystrophin. The dystrophin protein and other proteins associated with it provide structural support to the skeletal muscle membrane. Without it, muscles are more easily damaged during contraction. This damage promotes recruitment of immune cells which initiates the first stage of muscle repair. Under normal circumstances, this inflammatory reaction caused by immune cells restores the skeletal muscles. However, in DMD patients, repeated breakdown and regeneration of skeletal muscles leads to abnormal inflammation which promotes negative outcomes such as increased fibrosis. Fibrosis impairs muscle function, especially the diaphragm . Hamm et al., 2021 from the Grange lab investigated the effects of microdystrophin gene therapy and increased physical activity in mdx mice, a mouse model of DMD, with the idea that some of the negative changes with muscular dystrophy could be improved. The results showed a positive increase of endurance capacity in mdx mice treated with gene therapy alone (mdxGT group) and a greater increase if the mice also used a running wheel (mdxRGT group) compared to untreated mdx mice (mdx group). These findings suggested that gene therapy can increase a DMD patient's ability to become more physically active. However, the effects of running and microdystrophin gene therapy on the damaging inflammatory response in the diaphragm were not reported. To address this question, gene expression data from diaphragm muscles of all treatment groups were collected in the Hamm et al., 2021 study for later analysis. In my study, these diaphragm gene expression data were used to compare inflammatory signals between the various treatment groups. Indicators of skeletal muscle damage, immune cell accumulation and fibrosis deposition were rescued (i.e., returned to healthy mice levels) by microdystrophin gene therapy (mdxGT group). Running did not exert any negative effects on the majority of genes rescued by the microdystrophin therapy (mdxRGT group). These results indicated that voluntary wheel running could maintain the reduced inflammatory signals due to the microdystrophin gene therapy in mdx mice. If the function of the skeletal muscle of dystrophic boys was similarly improved by microdystrophin gene therapy and exercise did not interfere with its positive effects, DMD boys could potentially be physically active similar to normal boys of their age.